Autonomous implanted capsules are often called “leadless capsules” and distinguished from those electrodes or sensors placed at the distal end of a lead, which lead is traversed throughout its length by one or more conductors, connecting by galvanic conduction, the electrode or sensor to a generator connected at the opposite, proximal end, of the lead. Such leadless capsules are, for example, described in U.S. Patent Publication No. 2007/0088397 A1 and WO 2007/047681 A2 (Nanostim, Inc.) or in the U.S. Patent Production No. 2006/0136004 A1 (EBR Systems, Inc.).
Leadless capsules may be epicardial capsules fixed to the outer wall of the heart, or endocardial capsules fixed to the inside wall of a ventricular or atrial cavity. Their attachment to the heart wall is usually achieved by a protruding anchor such as a helical screw axially extending from the body of the capsule and designed to screw into and penetrate the heart tissue at the implantation site.
Leadless capsules typically include detection and/or stimulation circuitry to collect depolarization potentials of the myocardium and/or to apply stimulation pulses to the site where the leadless capsule is anchored, respectively. Such leadless capsules include an appropriate electrode, which optionally can be included in an active part of an anchoring screw. It can also incorporate one or more sensors for locally measuring the value of a parameter that is characteristic of the patient's physiological or physical condition, such as, for example, the patient's oxygen level in the blood, endocardial cardiac pressure, heart wall acceleration, and acceleration of the patient as an indicator of physical activity.
It should be understood, however, that the present invention is not limited to a particular type of leadless capsule, and is equally applicable to any type of leadless capsule, regardless of its functional purpose.
Of course, for a leadless capsule to exchange data with a remote device (i.e., in this context a device that is “remote” to the leadless capsule), the leadless capsule incorporates transmitter/receiver means for unidirectional and/or bidirectional wireless communications, as deemed appropriate for the purpose of the leadless capsule. Several techniques have been proposed for such wireless communications, in particular to allow a remote device to centralize the information collected by the leadless capsule and send, if necessary, appropriate instruction controls signals to the leadless capsule. As noted, the remote device in this instance may be an implanted pacemaker, defibrillator or resynchronizer, a subcutaneous defibrillator, or a long-term event recorder, and may be implanted or not implanted, and may be called a main device.
Thus, U.S. Patent Publication No. 2006/0136004 A1 proposes to transmit data by acoustic waves propagating inside the body. This technique is safe and effective, but it nevertheless has the disadvantage of requiring a relatively high transmission power, given the attenuation of acoustic waves into the body, and allows only relatively low data transmission rates.
U.S. Pat. No. 5,411,535 proposes a communication technique based on the use of radiofrequency (RF) waves. This also requires relatively high transmission power, and the attenuation of these waves by intracorporeal tissue is a major barrier to their spread.
Another technique proposed by U.S. Pat. No. 4,987,897 is a data exchange with a remote external device (programmer), through the skin, rather than only an intracorporeal transmission. This transmission is over a relatively short distance, between on the one hand, the housing of a pacemaker implanted in a subcutaneous pocket and, on the other hand, an external programmer placed against the skin near the generator. Currents therefore circulate through the skin in an area very distant from the sensitive areas, particularly in an area very distant from the myocardium, which avoids any risk of disruption of the natural or stimulated depolarization waves of the myocardium.
U.S. Patent Publication No. 2007/0088397 A1 proposes to use the stimulation pulses produced by a leadless capsule as a vehicle for the transmission of data previously collected or created by the leadless capsule. To this purpose, the pulse, instead of presenting a monotonic variation of voltage, is interrupted in a controlled manner for very short durations in order to create in the profile of the pulse very narrow pulses whose sequence corresponds to binary encoding of the information to be transmitted.
Whatever the communication technique used, the processing of the HBC signal collected at the leadless capsule requires significant energy compared to the energy resources available in the leadless capsule. Given its autonomous nature, the leadless capsule can in fact only use its own power resources, such as an energy harvester circuit (based on the movement of the leadless capsule) and/or a small integrated battery. The management of the available energy is thus a crucial point for the development of HBC techniques with and between autonomous leadless capsules.
It is recognized that the communication between a leadless capsule and a remote device is not continuous, and the active circuitry for signal processing (e.g., one or more of signal conditioning, amplification, scanning, filtering, decoding, and signal analysis) is therefore unnecessarily powered, with a negative impact on the autonomy of the leadless capsule and hence its useful lifetime, in the absence of a specific sleep implementation. By a “sleep implementation” it is meant that the relevant active circuits are powered down or off to conserve energy consumption (generally referred to as “sleep” or a “sleep mode”).
On the other hand, if the active circuits of the receiver are in a sleep mode then it becomes impossible to detect the occurrence of a HBC signal.
There is thus a need to “wake-up” the leadless capsules from a sleep mode when needed, with a relatively low latency to avoid burdening the flow of HBC signals or the responsiveness of the leadless capsule to the control signals that it receives by this HBC technique.
One known wake-up technique is to keep the active receiver circuits in a sleep implementation and wake them up at regular intervals to detect the presence of signals sent by another leadless capsule or another remote device. This technique requires a compromise between a low frequency of wake-ups—which saves energy but reduces the responsiveness—and conversely a higher frequency—which improves responsiveness but only slightly reduces the average energy consumption of the capsule in the long term.
US Patent Publication No. 2008/071328 A1 proposes an implantable leadless device equipped with a wake-up circuit activated by RF or acoustic signals (thus, not HBC signals) emitted from another remote device, these signals being possibly coded according, for example, to a particular addressing scheme allowing the wake-up of only one particular (or more) selected device. The use of non-HBC signals for the wake-up, however, requires the implantation of specific communication circuits both on the receiver and on the transmitter side of the communication.
The present invention, however, provides a different approach. It is based primarily on the use of a clock circuit that is a passive receiver (that is to say, a wake-up circuit that operates without an amplifier or any other circuit component having a significant energy consumption as compared to the overall energy balance of the leadless capsule) constantly able to detect the reception of a HBC signal. Only when such an HBC signal has been detected, does the clock circuit wake-up the system receiver circuit and activate the amplifiers and the other active circuits that need energy.
One of the major difficulties of this approach has been the low amplitude level of HBC signals to be detected by the wake-up circuit, given the rapid attenuation of propagating electrical pulses in the interstitial tissues of the body.
Another difficulty is that, the pulses of HBC signals can be mixed with parasitic electrical signals of relatively high level, such as the body's natural myopotentials and the cardiac depolarization waves propagating within the myocardium—hence there is a deteriorated signal/noise ratio. The stimulation pulses delivered by an implanted generator should also be discriminated so as to exclude them, which stimulation pulses are locally applied at specific sites of the myocardium, but then diffuse around the site of stimulation in a large area before being substantially attenuated.
It is desirable that the implemented technique should be able to address these issues and ensure effective screening and discrimination of all spurious signals.
As regards more particularly the signal attenuation by body tissues in the frequency range 500 kHz-10 MHz (band B), which is the one with minimal attenuation by the interstitial tissues of the body, the attenuation in this frequency range typically varies between 10 dB and 40 dB, depending on the distance between the transmitter and the receiver, the distance between the respective electrodes of the pair of electrodes of the transmitter or of the receiver, and the surface of these electrodes. A typical attenuation value is 20 dB at 1 MHz for a distance of 10 to 12 cm between the transmitter and the receiver.
But in extreme conditions, this attenuation can reach 60 dB, so that the clock signal sent to the leadless capsule is mixed in a background noise, thus having both a very low pulse amplitude to be detected and a very poor signal/noise ratio.